I think just replacing the HU is a great improvement if you can find one with a decent EQ. I went with the pioneer 3100. Its dd, steering wheel controls, usb, choice of different key colors, etc. Search for the thread.
I recommend you wire a new line to the new tweeter while you are doing the HU. I checked wiring on the dash (kickpanel) and it would be a lot easier just to wireup a new line.
Now I'm too lazy to take apart the stereo again lol. It sounds much better than stock already so I think most folks would be happy just swapping the HU.